Phase distribution in eutectic AuSn layer changed by temperature ramping rate and its effect on the performance of GaN-based vertical structure LEDs

AuSn bonding and laser lift-off (LLO) were employed to fabricate GaN-based vertical structure light emitting diodes (VSLEDs) on Si substrates. The phase distribution of ζ and δ phases in the eutectic AuSn layers was analyzed by electron back scattering diffraction (EBSD) measurements. It was found that the phase distribution changed regularly with the temperature ramping rate of the AuSn eutectic bonding. Raman scattering spectra indicated that more δ phase with good plasticity in the solder joint played an important role on the stress relaxation of the GaN. Light output power versus current (L–I) results showed that ζ phase penetration across AuSn layer enhanced the thermal and electrical conductivity, which could reduce the junction temperature and spread the current uniformly. The red shift of electroluminescence peaks under large current was caused by the large thermal mismatch stress in VSLEDs.